1. Field of the Invention
The present invention relates to a switching power source and, particularly, to a switching power source capable of reducing a standby power consumption of an electronic device as a load.
2. Description of Related Art
A conventional switching power source has been designed such that the power source efficiency becomes maximum for a load operating in a steady state. That is, the switching power source has been designed such that the power source efficiency becomes maximum for only a load of the, switching power source, which is operating normally and there has been no measure taken for power consumption when the load of the switching power source is light or the load is in a standby state. In view of this, it is usual to construct a power source with a main switching power source for a normal operation of a load and an additional switching power source such as a commercial power source transformer, the capacity of which corresponds to power required by the load in a standby state. With such power source construction, the cost of the power source for the load is increased and the size of the power source is also increased.
When the switching power source is used additionally, in order to reduce a switching loss during a standby state of a load, a switching frequency thereof during the standby state of the load is lowered. In such case, however, audible sound may be generated by the transformer.
In order to solve the problem of audible sound generation, a method for reducing power consumption when a load is light or the load is in a standby state has been proposed, in which an oscillator constituting the switching power source is controlled to oscillate intermittently while maintaining an oscillation frequency thereof, which is a switching frequency, unchanged. For example, Japanese Utility Model Application Laid-open No. H4-10588, Japanese Patent Application Laid-open No.3-243167, Japanese Patent Application Laid-open No. H4-12666 and Japanese Patent Application Laid-open No. H4-29482 disclose techniques belonging to such scheme.
However, in the techniques disclosed in Japanese Utility Model Application Laid-open No. H4-10588 and Japanese Patent Application Laid-open No.4-12666, the intermittent control of the oscillation state of oscillator is performed only when a load current is small. However, there is no description related to detail of such control. According to the technique disclosed in Japanese Patent Application Laid-open No. H3-243167, an interval of the intermittent oscillation control is determined by a CR time constant circuit. In the technique disclosed in Japanese Patent Application Laid-open No. H4-29482, the oscillation state is controlled intermittently on the basis of a voltage waveform of a commercial power source when a load is light.
In the techniques disclosed in these prior arts except that disclosed in Japanese Patent Application Laid-open No. H4-29482, the intermittent oscillation control is performed regardless of the voltage waveform of the commercial power source. Therefore, when the control is performed such that an oscillator oscillates during a time period other than a time period in which a charging capacitor constituting a primary side of a smoothing circuit for rectifying and smoothing a commercial power source voltage is charged, voltage ripple of the capacitor is increased, so that an output voltage on a secondary side of the smoothing circuit is varied.
Further, in the technique disclosed in Japanese Patent Application Laid-open No. H4-29482, the oscillation is stopped every half-wave rectified output of a commercial input power source although the intermittent oscillation control is performed correspondingly to the voltage waveform of the commercial power source. That is, the control is performed such that the oscillation is persistent for a half cycle of the commercial power source and is stopped for the other half cycle thereof. Therefore, the oscillation is persistent even for the period other than the period in which the charging of a primary side capacitor is performed and the above mentioned problem of the secondary side voltage variation is not solved.